This excerpt taken from the ATVI 10-Q filed Feb 8, 2006.
Correction of Bugs or Errors. Notwithstanding Microsofts Certification, all Software Titles must remain in compliance with all Certification requirements and requirements set forth in the Xbox 360 Publisher Guide on acontinuing and ongoing basis. Publisher must correct any material program bugs or errors in conformance with the Xbox 360 Publisher Guide whenever discovered and Publisher agrees to correct such material bugs and errors as soon as possible after discovery. With respect to bugs or errors discovered after Commercial Release of the applicable Software Title, Publisher will, at Microsofts request or allowance, correct the bug or error in all FPUs manufactured after discovery and Microsoft may charge a reasonable amount to cover the costs of Certifying the Software Title again.